1. Field of the Invention
The invention relates to a direct current (DC)-DC converter. Particularly, the invention relates to a DC-DC converter capable of adjusting a pulse width of a pulse width modulation (PWM) signal, and a voltage conversion method thereof.
2. Description of Related Art
It is known to all that operating voltages of a central processing unit (CPU), a dynamic random access memory (DRAM), a graphic chip, a chip set in a computer system are different, so that a plurality of direct current (DC)-DC converters are used to convert a DC input voltage (for example, 19V) provided by a power supplier into the operating voltages required by various components. A constant on time (COT) regulator is a DC-DC converter. Generally, when a feedback voltage is smaller than a reference voltage, the COT regulator can turn on a main switch in a fixed period, and can adjust a non-conducting period of the main switch to provide a regulated voltage.
When a load variation of the COT regulator is dramatic, an operating frequency of a pulse width modulation (PWM) signal of the COT regulator is correspondingly varied. For example, when the load of the COT regulator becomes lighter, the frequency of the PWM signal is degreased, and a distance between pulses is increased. When the load of the COT regulator becomes very light, the frequency of the PWM signal is lower than 25 KHz, by now the frequency falls into a frequency range that can be received by human's ear. Therefore, when the user uses an electronic product applying the COT regulator (for example, a mobile phone, a computer or a walkman, etc.), noise interference is occurred, which may decrease utilization quality of the electronic product.